Urban Design to Accommodate Trees: Introduction

Urban Design to Accommodate Trees: Introduction
by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville

Outline of topics Introduction Site evaluation Species selection
Formula for success Roots/hardscape conflicts Trees/sidewalk solutions Parking lot/buffer strip solutions Structural soils

Urban design to encourage tree canopy
Introduction Urban design to encourage tree canopy Trees often grow poorly in urban areas unless the infrastructure has been specially designed to accommodate tree root growth This presentation is designed to help guide you through the design and species selection process Occasionally, portions of cities get lucky and trees grow despite poor planning.

Few citizens want cities without trees
Introduction Few citizens want cities without trees A city without trees is hotter in summer, receives less rainfall, has greater runoff following storms, has fewer shoppers, and is not inviting Citizens have decided in most communities that this does not work.

Poor design leads to failure
Introduction Poor design leads to failure Trees struggle unless spaces are designed appropriately When lots of money is thrown at tree projects without guidance from knowledgeable professionals, waste occurs and no one wins Here is a $700 tree planted in a space that cost several thousand dollars. The money was not spent wisely.

Good design leads to success
Introduction Good design leads to success When money is spent wisely under the guidance of qualified urban foresters, good things can happen. Most people prefer to look at the view above than a city view without trees. Trees thrive when good designs are executed properly Healthy trees increase property value, intercept air pollutants, buffer temperatures, reduce wind speed, cool the city, reduce runoff from storms, encourage people to visit and spend money at shops, and create a more inviting community

Site evaluation A thorough site evaluation insures that you will select the right tree for your planting site

Examples of some of the components of site evaluation
Above ground USDA hardiness zone Light, heat, and wind exposure Below ground Soil volume – is there enough root space? Soil pH and drainage Soil texture, compaction Maintenance issues Availability of regular irrigation Pruning program in place or not

Other important site evaluation criteria
What is the average annual rainfall in the area? Will the tree be planted:in the ground, in containers or in above ground planters, or near the coast What is the distance between the top of the water table and the soil surface? How will the site be irrigated?. Will the tree be planted in a tree lawn or streetscape (the grassy strip between the curb and the sidewalk)? Will the tree be planted along a street without a sidewalk. Will the tree be planted in a sidewalk cutout? Will the tree be planted in a parking lot? Will the tree be planted in an open lawn area or in a shrub bed? What is the approximate size of this area? Will the tree be planted within 8 feet of a sidewalk, driveway or other hard surface? Will an adjacent sidewalk or roadway receive deicing salts? Is there a swimming pool, vegetable garden, masonry wall or septic tank or drain field within 50 feet of the planting site? Are overhead wires within 30 feet of the planting site? Is there a street light or security-type light within 35 feet of the planting hole? Is the planting site within 35 feet of a building? Would you care to eliminate trees that could drop messy fruit, large leaves or twigs during an extended period? Would you like to eliminate trees that are known to be susceptible to breakage? What is your budget for pruning trees? Would you care to plant only native trees? Please list any other attributes that you would like your trees to have? Other considerations: 1) soil salt levels, 2) soil contamination, 3) soil layering, presence of construction debris, 4) health of and type of existing plants, 5) presence of underground utilities, 6) ordinance restrictions, 7) species diversity index, 8) politics, 9) community expectations, 10) design life of site, 11) tree life expectancy, 12) forest vs. former agricultural field, 13) location of existing tree roots, 14) recent construction activities.

Formula for success Roots/hardscape conflicts Trees/sidewalk solutions Parking lot/buffer strip solutions Structural soils The site has been evaluated. Now it is time to chose trees for the site. Many professionals will tell you that politics has lots to do with species selection. In at least some situations, politics overshadows process of site evaluation and matching species to the site characteristics. This can lead to problems if the politically chosen trees are not suited for the conditions at the site.

Selection The dilemma …the design… Certain trees grow well in tough urban sites so we use them often..monoculture results They grow well in small spaces but disrupt and destroy sidewalks/curbs, grow into wires We “fix” the problem by cutting roots and resurfacing hardscape, or cutting tops Trees decline or look ugly as a result and…… …..our vision of the urban forest never develops because trees never make it more than 20 to 40 years We can do better with appropriate design

The dilemma continued …the trees… We could try different species or cultivars but they may perform poorly and besides “no one else has tried these” And alternative trees may be difficult to find at nurseries, especially in the size and quantity you want So……we plant what we know will work; i.e. what everyone else plants, because it is safe We are more or less stuck in this pattern now

Solution – be creative Restrict one genera or species to < 20% for few years Develop a list of alternatives for each commonly planted tree For example alternatives to live oak: Swamp chestnut, redbay, trident maple, sugarberry, ash, sweetgum, american elm, cedar elm, overcup oak,

Match species to site characteristics
Selection Choose the right tree that will grow in the conditions present at the site: use books, software, web sites, your experience …or… Design the right place to fit the trees you want: this is covered in detail next These two options above are very different from each other – explain that to the audience. In addition, certain trees such as elms, honeylocust, live oak, bradford pear and other urban tolerant trees are able to grow just about anywhere in urban areas. Their roots are able to expand into even the smallest of soil spaces by exploiting cracks and crevasses in compacted soil. These trees often grow well in small soil spaces, but can lift hardscape in the process. So there is the dilemma: these tough trees can grow in small spaces but disrupt infrastructure in the process. Roots are often cut to “fix” the hardscape problem leading to tree decline. As a result, the urban forest does not develop into the dream you envisioned. So do you design larger soil spaces for urban tough trees to prevent hardscape damage? I guess that depends on your time frame. If you have a long term view and want to design sustainable landscapes that support large trees, you might choose to design in more soil space. If you have a short term view and are not concerned about developing an urban forest with large shade trees then do what most folks do-design for the 20-year planned obsolescence of the landscape. Trees that are not as urban tough, such as red maples, require larger soil spaces to even survive and grow well in the urban landscape. If large soil space for root expansion is not included in the design these trees often fail before 20 years. Don’t try to shoehorn a tree you want into a site not designed to support that tree, unless you are a short term planner, in which case go for it

Examples of right tree in the right place
Selection Examples of right tree in the right place Compacted soil = surface roots and hardscape damage likely so pick small trees tolerant of low oxygen Wires or lights overhead = regular pruning so pick small trees or move wires and lights Poor drainage = surface roots so pick small trees or design in more soil space Narrow planting strip = deflected roots and toppled trees so pick small trees or design in more space Small planting pit = hardscape damage and poor growth so pick small, low O2 trees Parking lot island = drought, small space, heat so pick urban tough, drought tolerant trees Park/campus/lawn = plenty of soil space so pick large maturing trees Soil pH 8.2 = poor growth on many trees so pick alkaline soil tolerant trees

Diversity can be the key to adversity
Selection But, once it is discovered that a certain tree grows well in a situation, it tends to be used over and over again….what is wrong with that? Perhaps nothing so long as a pest problem like Dutch elm disease, emerald ash borer, and others do not occur. How lucky do you feel? Detroit is not feeling too lucky. Your community has to make that decision Examples of trees that we have used over and over again because they have grown well in urban areas include American elm, honeylocust, eucalyptus, Bradford Callary pear, ashes, red oak and others. All of these have developed major pest problems that are causing (or have caused) havoc in some communities.

Species selection for hardscape compatibility
Trees that have a long life span generally compartmentalize decay well so they are resilient Trees that are free of serious pest problems are good Trees that cause damage tend to be those that grow to a large size. Although planting small-statured trees can help to minimize damage to curbs and sidewalks, small trees provide few of the benefits to the community that large trees provide.

Trees that develop a prominent root flare (swelling at the base of the trunk) can lift sidewalks and curbs soon after planting in many circumstances if special provisions are not incorporated into the design Trees that cause damage tend to be those that grow to a large size. Although planting small-statured trees can help to minimize damage to curbs and sidewalks, small trees provide few of the benefits to the community that large trees provide.

Avoid planting trees with fleshy fruit to reduce the likelihood of people slipping and falling on walks and pavement This is likely to be more of an issue where sidewalks are under the canopy

Avoid trees with long sharp thorns or spines unless the thorns will be well above the ground and out of the way of pedestrians Trees with thorns can make for wonderful barriers but use them with caution near walks and streets where pedestrians come in contact with the tree.

Trees tolerant of low soil oxygen conditions often perform better than other trees when placed in small restricted soil spaces with poor drainage Ashes, baldcypress, certain oaks, and many other wet site tolerant trees often perform well near hardscape due to their tolerance of low soil oxygen levels.

Other selection criteria
Showy flowers/bark Canopy density/texture Attracts wildlife/or not Leaf size/messiness Nice fall color Single/multi-trunked Pruning requirement Canopy form/habit And others….

Formula for success Roots/hardscape conflicts Trees/sidewalk solutions Parking lot/buffer strip solutions Structural soils Here are several examples of what has worked in urban and suburban landscapes. Look carefully at each photograph to see that the key to success is designing adequate soil space into the system. You might ask the audience if they can determine what is similar among all the examples before you tell them that the answer is adequate soil space for root expansion.

Trees can form a canopy over the street
Success Trees can form a canopy over the street With appropriate spacing Access to open soil space Open soil space is soil that is not covered by a hard surface such as a sidewalk, pavement or a building Notice that in all of the following cases, the trees have access to large volumes of soil for root expansion.

Complete canopy closure
Success Complete canopy closure Trees were planted 40 to 50 feet apart in a planting strip 10 feet wide; this spacing allowed for the crowns of individual trees to touch, encouraging development of a more natural upright form The 10' wide planting strip allowed the trunk flare to develop appropriately State College, Pennsylvania has managed to maintain some of its population of American elms with regular disease-monitoring programs. Part of the success can be attributed to the adequate soil space for root expansion. Roots can grow under the sidewalks and into the lawns of the homes along this street. This has allowed them to gain tremendous size. Note the location of the street lights-under the canopy. This is the way to develop an urban forest. State College, Pennsylvania

Success Complete canopy closure Trees were planted about 30 feet apart; this spacing allowed for the crowns of individual trees to touch when they were fairly young and encouraged a more natural upright form Trees gained tremendous size due to the almost unlimited access roots had to soil space St. Augustine Florida planted these live oaks along the street in the lawns of homes on the right and between the curb and a wall on the left. The natural upright form of the trees helps minimize maintenance needs by reducing the pruning required to keep trees clear of pedestrians, vehicles and buildings. This is the way to develop an urban forest. Saint Augustine, Florida

Success Complete canopy closure Trees were planted 15 to 40 feet apart; this spacing allowed for the crowns of individual trees to touch when they were fairly young encouraging a more natural upright form Trees gained tremendous size due to the almost unlimited access roots had to soil space Seattle, Washington planted four rows of big leaf maples along the street and in the lawns of this campus. The natural upright form of the trees helps minimize maintenance needs by reducing the pruning required to keep trees clear of pedestrians, vehicles and buildings. This is the way to develop an urban forest. Seattle, Washington

Barely complete canopy closure
Success Barely complete canopy closure Trees were planted about 50 feet apart. Because trees were spaced this far apart, they began to grow aggressive lower limbs. Lower limbs are drooping, creating a more spreading habit than would have occurred with closer spacing Miami, Florida planted live oaks along the street and set them back about 8 feet from the edge of the road. Roots were able to grow into adjacent lawns of the homes along this street. Although the spreading habit is fine for parks and perhaps other places where vehicles and pedestrians do not have to pass under the canopy, it has little place in the urban and suburban landscape. The lower limbs must be pruned or removed to allow passage. This increases maintenance costs and can encourage an unhealthy urban forest by initiating decay in the large pruning cuts that are necessary. Plant trees closer than this in order to create an urban forest. Miami, Florida Trees gained tremendous size due to the almost unlimited access roots had to soil space

No canopy closure— spacing too far
Success No canopy closure— spacing too far Trees were planted about 50 feet apart. Because trees were spaced this far apart, they began to grow aggressive lower limbs. Lower limbs are drooping, creating a more spreading habit than would have occurred with closer spacing Trees gained tremendous size due to the almost unlimited access roots had to soil space Charleston, South Carolina planted these trees ten feet from the curb in a downtown park. The lower limbs must be pruned or removed to allow passage. This increases maintenance costs and can encourage an unhealthy urban forest by initiating decay in the large pruning cuts that are necessary. Plant trees closer than this in order to create an urban forest. Charleston, South Carolina

Success No canopy closure Trees were planted about feet apart. Because trees were spaced this far apart, they began to grow aggressive lower limbs. The planting strip is twenty feet wide and roots can grow into the lawns of the homes along the street Ficus trees with plenty of soil space for root expansion. If you provide the space for roots to grow trees are likely to be healthy and grow to a large size. The lower limbs must be pruned or removed to allow passage. This increases maintenance costs and can encourage an unhealthy urban forest by initiating decay in the large pruning cuts that are necessary. Plant trees closer than this in order to create an urban forest. Coral Gables, Florida

The formula Plenty of root space
Closer spacing for canopy closure and reduced maintenance

Roots can destroy hardscape with improper design
Conflicts Roots can destroy hardscape with improper design Tree roots grow under sidewalks and asphalt in many instances because that is where the soil oxygen and moisture are located The hardscape is often inadvertently designed to encourage roots to grow there; better urban design can reduce the likelihood of roots proliferating under hardscape

Root spread on shade trees
Conflicts Root spread on shade trees Shade trees extend their roots way beyond the tree canopy Note the root that is growing in the lawn (two arrows); it is located well beyond the branch tips Roots typically spread way out beyond the edge of the canopy

Roots grow well beyond canopy edge
Conflicts Roots grow well beyond canopy edge Trees that normally grow a very expansive root system can become stressed and grow poorly in urban landscapes where soil space is limited The result can be poor tree health, damaged sidewalks and curbs, and other problems Limiting the space for root growth places a tremendous amount of stress on the tree.

Root flare needs room to expand
Conflicts Root flare needs room to expand The swelling at the base of the tree (where the large roots meet the trunk) is commonly referred to as the root flare or buttress Roots normally raise out of the ground as shown here Adequate open soil space must be designed into the system to accommodate expansion of the root flare Some trees produce huge buttress roots and prominent root flares at the base of the trunk; others never develop a swelling or root flare. Flare commonly 2.5 to 3.5 times trunk diameter

Misfits and poor design
Conflicts Misfits and poor design The oaks planted in this narrow soil strip have two choices: grow poorly due to the limited amount of soil space available for root expansion, or grow well by sending roots under the pavement which will quickly crumble the curb and asphalt It is difficult to understand why this poor design is still so common across the US.

Misplacement of large maturing trees
Conflicts Misplacement of large maturing trees The honeylocust planted between the walk and the wall are capable of growing to a large size. In order to thrive in this site, the trees’ roots will have to grow under the wall and into the lawn behind the wall The wall is likely to be displaced as the root flare develops and the roots expand in diameter beneath the wall The wall will probably be damaged because roots that grow under the wall typically do so just under the footing. Installation of a root barrier, or perhaps a 6-inch-deep layer of washed 3/4" to 1" gravel installed under the footing might prolong the life of the wall.

Young trees likely to grow to disrupt hardscape
Conflicts Young trees likely to grow to disrupt hardscape The trees planted in this three to four foot wide strip are likely to cause disruption to the curb, sidewalk, and driveways along this street These repairs cost communities in the US approximately 2 billion dollars annually This well-intentioned project could have made the same impact by attaching the sidewalk to the curb. This would have encouraged root growth away from the sidewalk because the trees would have been placed on the other side of the walk. This would give the roots easy access to the soil in the landscape beds.

Conflicts Damage can result Large maturing trees located too close to walks can cause structural damage that is costly to repair

Conflicts Sidewalks lifted Roots often grow just under the slab because that is where moisture and oxygen are abundant Roots lift the walk as they grow in diameter Large maturing trees located too close to walks can cause structural damage that is costly to repair.

Picking slow growing trees can help
Conflicts Picking slow growing trees can help Trees that remain small at maturity often cause less damage than large trees More small trees will be required (at a spacing of approximately 25 feet) to develop a closed canopy than if large maturing trees were planted The disadvantage to this strategy is that small trees typically are shorter lived than large trees.

Formula for success Roots/hardscape conflicts Trees/sidewalk solutions (go to sidewalk solutions PP file) Parking lot/buffer strip solutions Structural soils

Urban Design to Accommodate Trees: Introduction
by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville

Urban Design to Accommodate Trees: Introduction

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